Thermoplastic welding is a process by which plastic parts are fusion bonded together to form an integral part. As used herein, the term "plastic parts" includes composite thermoplastic parts, and thermoset composite parts with resin rich thermoplastic surface layers. This process offers considerable promise as a manufacturing technique because it eliminates the need for fasteners which are traditionally used to fasten parts together into assemblies. The installation of fasteners is a expensive and time consuming process and potentially weakens the structure because of the holes necessarily drilled into the structure for installation of the fasteners. Without special care to harden or reinforce the peripheral regions around these holes, they can serve as the origination points for stress cracking and can also be instrumental in the beginning of corrosion problems and leaks. Parts which are fastened together by conventional fasteners in the aerospace industry in particular must often have a sealant applied not only to the fastener holes but also to the faying surfaces of the parts between the fasteners to seal the faying surfaces against leakage of water from the outside and against loss of air pressure from the inside, in the case of a pressurized hull. Aircraft wing boxes which double as fuel tanks in aircraft must also be carefully sealed to prevent leakage of fuel from the wing box, even when the wing box flexes in operation as wing boxes are normally designed to do, and to prevent arcing from metal fasteners.
Thermoplastic welding involves the application of heat to the plastic parts to raise the temperature of the faying surfaces to the temperature at which the thermoplastic can melt and flow together. For most applications, and in particular for structural applications, it is preferably to concentrate the heat application at the bond line to avoid heating the entire structure. Heating an entire structure made of thermoplastic composites is disadvantageous because it can cause the structure to lose its rigidity and its shape, and possibly incur some delamination, unless complicated tooling is provided to support the structure against sagging under temperatures close to its melting point. One technique for applying heat directly to the faying surfaces of plastic parts to raise the temperature to the melting point for fusion bonding is with induction heating. Electrically conductive materials can be heated resistively by small scale eddy currents in the material induced by an alternating magnetic field generated by an induction coil. A foraminous metallic bond line susceptor is one known material receptive to the induction of eddy currents under the influence of an induction coil.
A fusion bond between two plastic parts is inherently uninspectable by known methods of non-destructive testing. The fusion zone is buried between the two parts and the only reliable method for determining whether the parts are completely bonded is to cut the bonded parts into sections and subject the sections to destructive tensile testing. Other techniques which are non-destructive, such as ultrasonic echo analysis, are effective for discovering gaps in the faying surfaces, but when unbonded surfaces are in contact, the ultrasonic echo techniques may not reveal the existence of an unbonded zone.
The traditional approach in manufacturing for dealing with uninspectable product regions is to develop a process for producing products which are extensively tested to destruction to empirically establish the process limits within which the process yields acceptable products. Then, with rigid process controls and periodic confirmation testing, confidence is established in the integrity of the process even though the assemblies that are produced by the process cannot be non-destructively tested to confirm that the process is producing the desired products. Basically, the theory is that if the assembly is produced within the same range of significant production parameters used to produce the earlier assemblies which yielded acceptable test results, any product variation between the tested product and the later untested products will have already been shown to be insignificant.
In the case of thermoplastic fusion bonding, the significant process parameters include the pressure exerted on the parts, the thickness and material characteristics of the top part, the speed at which the coil is moved over the bond line, the surface fit of the faying surfaces of the two parts, the power and frequency applied to the coil, the temperature and dwell time at temperature at the bond line, and the separation between the coil and the bond line. These process parameters are typically difficult to control to the precision desired for accurate process control and are also particularly difficult to record and correlate with the zone tested on the parts after the process has been performed. For example, after the parts are sectioned and subjected to tensile tests, it would be useful to correlate the regions of incomplete welding with the process parameters at that particular zone when the process was being performed. In this way, a map can be established corresponding to the faying surfaces of the parts describing the process parameters in affect at each point along the faying surface. Problem areas can then be identified and the process parameters at those problems areas can be analyzed in detail for process improvement. Afterward, after the process is perfected, a permanent record can be established for each part showing the process parameters in effect at every point along each of the bond lines in the assembly so that a complete manufacturing record can be created for each assembly as required by certain regulatory regimes in the manufacture of flight critical hardware.
Thus, there is a need in the developing art of thermoplastic bonding of parts into large scale assemblies for a method and apparatus for reliably and repeatably performing an inductive welding process within a fixed range of process parameters and for affording the possibility of sensing and recording the process parameters for statistical process control and as a manufacturing record of the parts produced.